In recent years, wavelength tunable semiconductor lasers have been expected to be used as light sources or local oscillators in receivers in coherent optical systems or in wavelength division multiplexing systems in which light beams of different wavelengths are multiplexed to increase transmission capacity. Among various types of wavelength tunable lasers, distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers which do not employ external reflectors but include diffraction gratings have been extensively developed.
FIG. 19 is a perspective view illustrating a prior art wavelength tunable semiconductor laser disclosed in, for example, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 5, No. 3, March 1993, pp. 273-275, "Tunable Twin-Guide Lasers with Improved Performance Fabricated by Metal-Organic Vapor Phase Epitaxy" T. Wolf, S. Illek, J. Rieger, B. Borchert, and W. Thulke. FIG. 20 is a sectional view perpendicular to the resonator length direction of the laser shown in FIG. 19.
In these figures, reference numeral 201 designates a p type InP substrate. A p type InP buffer layer 202 having a stripe-shaped ridge portion is disposed on the p type InP substrate 201. A tuning layer 203 is disposed on the ridge portion of the buffer layer 202. The tuning layer 203 comprises InGaAsP in a composition ratio equivalent to a band gap wavelength .lambda.g of 1.3 .mu.m. An n type InP spacer layer 204 is disposed on the tuning layer 203. An active layer 205 is disposed on the spacer layer 204. The active layer 205 comprises InGaAsP in a composition ratio equivalent to a band gap wavelength .lambda.g of 1.55 .mu.m. A p type InP guide layer 206 is disposed on the active layer 205. A diffraction grating layer 207 is disposed on the guide layer 206. An n type InP layer 209 is disposed on the buffer layer 202, contacting both sides of the ridge structure comprising the above-described layers 202 to 207. P type InP buffer layers 208b are disposed on the ridge structure and on the n type InP layer 209 except for a stripe-shaped region on the n type InP layer 209. P type InGaAsP contact layers 210 are disposed on the p type InP buffer layers 208b. An insulating film 211 is disposed on the surface of the structure. The insulating film 211 has an opening 211a on the contact layer and an opening 211b on the n type InP layer 209. A p side electrode 213 for laser operation contacts the contact layer 210 through the opening 211a of the insulating film 211, and a common n side electrode 212 contacts the n type InP layer 209 through the opening 211b of the insulating film 211. A p side electrode 214 for wavelength tuning is disposed on the rear surface of the substrate 201. An arrow 220 shows a current injected from the p side electrode 213 and producing laser light emission. An arrow 221 shows a reactive current that does not produce laser light emission. An arrow 222 shows a wavelength tuning current injected from the p side electrode 214.
This prior art wavelength tunable semiconductor laser is called a TTG (Tunable Twin-Guide) structure. Among TTG lasers, a TTG laser in which current is injected into a tuning layer (hereinafter referred to as a current-injected type TTG) provides the widest range of continuous wavelength tuning, theoretically.
A description is given of a method of fabricating this prior art laser.
Initially, the p type InP buffer layer 202, the InGaAsP tuning layer 203, the n type InP spacer layer 204, the InGaAsP active layer 205, the p type InP guide layer 206, and the diffraction grating layer 207 are successively grown on the p type InP substrate 201 using metal-organic vapor phase epitaxy (MOVPE). Then, the diffraction grating layer 207 is patterned by photolithography and etching to form a plurality of stripes periodically arranged in what becomes the resonator length direction of the laser, i.e., the light propagating direction. Thereafter, the p type InP buffer layer 208a is grown to bury the periodic stripes. Next, an SiO.sub.2 film is deposited on the p type InP buffer layer 208a by sputtering and patterned in a stripe shape extending in the resonator length direction using photolithography and reactive ion etching (RIE). Using this stripe-shaped SiO.sub.2 film as a mask, portions of the semiconductor laminated structure from the p type InP buffer layer 208a to the p type InP buffer layer 202 are etched away by RIE, forming a ridge-shaped waveguide (hereinafter referred to as a ridge waveguide) in which the width of the active layer is 1.2 .mu.m. Using the stripe-shaped SiO.sub.2 film as a mask for selective growth, the n type InP layer 209 is grown on the p type InP buffer layer 202 at opposite sides of the ridge waveguide. After removal of the SiO.sub.2 film, the p type InP buffer layer 208b is grown on the n type InP layer 209 and on the p type InP buffer layer 208a and, successively, the p type InGaAsP contact layer 210 is grown on the buffer layer 208b. Thereafter, portions of the contact layer 210 and the buffer layer 208b are selectively removed until the surface of the n type InP layer 209 is exposed, followed by deposition of an insulating film 211, such as SiO.sub.2, over the entire surface of the structure. Then, the opening 211a is formed in the insulating film 211 to expose the surface of the p type InGaAsP contact layer 210 opposite the ridge waveguide and the opening 211b is formed in the insulating film 211 to expose the surface of the n type InP layer 209. The upper p side electrode 213 is formed in contact with the exposed surface of the p type InGaAsP contact layer 210 and the n side electrode 212 is formed in contact with the exposed surface of the n type InP layer 209. Finally, the lower p side electrode 214 is formed on the rear surface of the p type InP substrate 201 to complete the TTG laser structure shown in FIG. 19.
A description is given of the operating principle.
As shown in FIG. 20, the current 220 contributing to the generation of laser light is supplied from the p side electrode 213 and flows through the p type InGaAs contact layer 210, the p type InP buffer layer 208, the p type InP guide layer 206, the active layer 205, the n type InP spacer layer 204, and the n type InP layer 209 to the n side electrode 212. On the other hand, the wavelength tuning current 222 is supplied from the p side electrode 214 and flows through the p type InP substrate 201, the p type InP buffer layer 202, the tuning layer 203, the n type InP spacer layer 204, and the n type InP layer 209 to the n side electrode 212. In this way, in the current-injected type TTG structure, the active layer 205 and the tuning layer 203, which layers sandwich the n type InP spacer layer 204, are individually controlled. Further, in the TTG structure, a main portion of the electric field of the laser light generated in the active layer 205 extends to the diffraction grating layer 207 and the tuning layer 203. Therefore, when the tuning current applied to the tuning layer 203 is varied while maintaining the laser driving current applied to the active layer 205 constant, i.e., while maintaining the gain of the laser light constant, the refractive index of the tuning layer 203 varies due to the plasma effect and the equivalent refractive index responded to by the photons varies, whereby the oscillation wavelength of the laser light is tuned.
Now assuming that the oscillation wavelength of the laser light is .lambda. and the equivalent refractive index is n.sub.eff, the relationship between .lambda. and n.sub.eff is represented by EQU .lambda.=2n.sub.eff .LAMBDA.
where .LAMBDA. is the period of the diffraction grating.
When the variation in the equivalent refractive index due to the current injection into the tuning layer 203 is .DELTA.n.sub.eff, the variation in the wavelength .DELTA..lambda. is represented by EQU .DELTA..lambda.=2.DELTA.n.sub.eff .LAMBDA.
In the above-described literature, when the laser driving current applied to the active layer 205 is 120 mA and the tuning current applied to the tuning layer 203 is 50 mA, a wavelength variation of 4.7 nm is obtained while maintaining the maximum laser output of 3 mW. When the laser driving current is 60 mA and the tuning current is 70 mA, a widest continuous tuning range of 5.1 nm is obtained.
However, when the refractive index of the tuning layer 203 is varied utilizing the plasma effect caused by the current injection into the tuning layer 203 as described above, injected carriers recombine at random, whereby the carrier concentration fluctuates, resulting in an increase in the spectral width of the laser light. Although the spectral width must be lower than several MHz in wavelength division multiplexing systems, the spectral width unfavorably increases with an increase in the wavelength tunable range. For example, in the prior art wavelength tunable semiconductor laser described in the literature, the spectral width varies in a range from 5.4 MHz to 50 MHz in response to the wavelength tunable range, and the spectral width becomes 26.5 MHz when the tunable range is 3 nm.
In order to prevent the unwanted increase in the spectral width, a reverse electric field is applied to the tuning layer to change the refractive index of the tuning layer, using the Franz-Keldysh effect, when the tuning layer is a bulk layer or using the quantum confined Stark effect when the tuning layer is a multiquantum well layer. For example, Applied Physics Letters 59(21), 18 Nov. 1991, pp. 2721-2723 "Optical modulation characteristics of a twin-guide laser by an electric field" and Applied Physics Letters 60(20), 18 May 1992, pp. 2472 to 2474 "Tunable twin-guide lasers with flat frequency modulation response by quantum confined Stark effect" disclose such electric field applied type TTG semiconductor laser.
FIG. 21 is a diagram for explaining the operation of the electric field applied type TTG semiconductor laser. The structure of this electric field applied TTG semiconductor laser is identical to the structure of the current injected type TTG semiconductor laser shown in FIG. 19. FIG. 21 is a sectional view of the waveguide part of the electric field applied type TTG laser along the resonator length direction. In the figure, the same reference numerals as in FIG. 19 designate the same or corresponding parts. Reference numeral 50 designates a power supply for supplying a laser driving current to the active layer 205. A resistor 51 is connected between the power source 50 and the laser element in series. Reference numeral 40 designates a variable power supply for supplying an electric field to the tuning layer 203. A resistor 41 is connected between the power supply 40 and the laser element in parallel. The power supply terminals connected to the spacer layer 204 in FIG. 21 are actually connected to the n side electrode 212 shown in FIG. 19.
A description is given of the operation.
In the electric field applied type TTG laser, a laser driving current from the power supply 50 is applied through the resistor 51 to the active layer 205 to produce a laser light and, simultaneously, a reverse bias voltage is applied across the n side electrode 212 and the lower p side electrode 214 by the variable power supply 40 and the resistor 41 to apply a reverse electric field to the tuning layer 203, whereby the refractive index of the tuning layer 203 is varied utilizing Franz-Keldysh effect or quantum confined Stark effect. In this way, the wavelength of the laser light is tuned.
In this electric field applied type TTG laser, since the fluctuation of the carrier concentration due to the random recombination of carriers in the tuning layer does not occur, the above-described problems of the current injected type TTG laser are avoided.
However, in the electric field applied type TTG layer, as mentioned in the above-described literatures, i.e., Applied Physics Letters 59(21), 18 Nov. 1991, pp. 2721 to 2723 and Applied Physics Letters 60(20), 18 May 1992, pp. 2472 to 2474, a wide wavelength tuning range cannot be obtained for the reasons described hereinafter.
That is, in the structure shown in FIG. 21, when a reverse electric field is applied to the tuning layer 203, the refractive index of the tuning layer 203 varies as shown by a curve k' in FIG. 22(b). Simultaneously with the variation in the refractive index, the magnitude of light absorption in the tuning layer 203 varies as shown by a curve k in FIG. 22(a) due to the Kramers-Kronig relations.
In FIG. 22(b), in the proximity of the laser oscillation wavelength of 1.55 .mu.m, the variation in refractive index .DELTA.n1 due to the application of the reverse electric field to the tuning layer 203 is larger than 0, that is, the refractive index increases in variation. On the other hand, when the magnitude of absorption in the tuning layer 203 increases by .DELTA..alpha.2 as shown in FIG. 22(a), the threshold current required for the laser oscillation is increased, i.e., the carrier concentration required for laser oscillation is increased, whereby the carrier concentration in the laser resonator is increased. Therefore, a negative variation in refractive index .DELTA.n2 (.DELTA.n2&lt;0) in which the refractive index decreases is caused by the plasma effect. This negative variation in refractive index .DELTA.n2 cancels the above-described positive variation in refractive index .DELTA.n1 due to the application of the reverse electric field, so that the variation in refractive index of the whole laser resonator is reduced. As a result, the obtained wavelength tunable range is reduced.
As described above, although the prior art current injected type TTG laser provides a wide wavelength tunable range, the current injection causes noise. Therefore, it is difficult to obtain a sharp spectrum. Further, since the spectral width increases with an increase in the wavelength tunable range, adjacent spectrums affect each other when laser light is multiplexed and transmitted, resulting in poor transmission performance and poor practicability.
On the other hand, when an electric field is applied to the tuning layer of the above-described TTG laser in order to prevent the unwanted increase in the spectrum width, the variation in refractive index is reduced as a result of an increase in the magnitude of absorption in the tuning layer caused by the application of the electric field. Therefore, a wide wavelength tunable range cannot be obtained.